Aqueous coating compositions are a now-established substitute for solventborne systems. One important class of aqueous binders are the polyurethane dispersions (D. Dieterich, Prog. Org. Coatings 9, 281(1981)), which unite the most important qualities of resistance towards chemicals and mechanical loading. Especially in the area of the coating of surfaces subject to severe mechanical stress, therefore, the use of polyurethane dispersions is an advantage.
In respect especially of the chemical resistance a further improvement can be achieved by chemically crosslinking the coatings. One way of achieving this is the crosslinking of aqueous binders which carry OH or NH groups by means of hydrophilized polyisocyanates (EP-B-0540985). Aqueous 2 k (two-component) polyurethane coating materials of this kind attain a very high level of properties, but the application procedure is comparatively costly and inconvenient and may represent a barrier, particularly in the case of artisanal use.
Another way is to use UV-crosslinkable polyurethane dispersions, with which likewise excellent properties can be attained. Such binders generally contain activated double bonds, such as acrylate groups, for example (EP-B-0753531), and are established in particular in the industrial coating of sheet-like substrates. Not in every application, however, is it practicable to perform crosslinking with actinic radiation following application.
A likewise established way, therefore, is to use polyurethane dispersions having non-activated double bonds, which have air-drying properties (Advances in Urethane Science and Technology, K. C. Frisch, D. Klempner (Eds.) Vol. 10, p. 121 ff. (1987)). Coatings comprising such binders crosslink through reaction with atmospheric oxygen (auto-oxidative crosslinking) and can therefore be employed in one-component form, and crosslink without further input by the user.
Dispersions of this kind containing urethane groups and air-drying components are further described in EP-A-0017199, EP-B-0379007, WO199719120, DE-A-4416336, U.S. Pat. No. 5,039,732 and JP-A-6340842. A disadvantage of these auto-oxidatively crosslinkable polyurethane dispersions, however, is that the mechanical properties of the coating film do not achieve the high level of the purely physically drying polyurethane dispersions. This is manifested, for example, in impaired abrasion resistance.
An improvement in the abrasion resistance can be achieved by systems based on modified, OH-functional triglycerides. One such aqueous polyurethane binder is described in EP-B-0709414. In that case dehydrated castor oil is used. As a result of the dehydration, OH groups of the ricinoleic acid are converted into double bonds, and so the sum total of the OH groups and double bonds always remains the same.
More flexible, therefore, is the process presented in EP-B-1198487, in which the castor oil is mixed with another, non-functional, triglyceride and the mixture is then subjected to transesterification. Polyurethane dispersions based on this process already have very good properties in terms of mechanical and chemical resistance, and can be employed very effectively in, for example, high-grade floor coatings. Nevertheless, they still do not achieve the particularly high level of properties of radiation-curable polyurethane dispersions or of 2 k polyurethane dispersions, each of which achieve much higher crosslinking density.